Conventional internal combustion engines such as those used in automobiles and trucks have various characteristics which result in relatively low efficiency coupled with unacceptable levels of exhaust emissions. For example, the power delivered by such an engine at any speed is regulated by controlling the amount of air-fuel mixture entering the cylinder, and during all part load operation energy is lost in the process of inducting air through the carburetor past the partially closed throttle. This also decreases the intake manifold pressure below atmospheric, with consequent variation of air-fuel ratios, compression ratios, heat rejection and flame speed.
Moreover, at all engine speeds the charge in the cylinder contains some exhaust gas from the previous cycle left in the combustion chamber. At idle or light load conditions this residue may be as must as one-third of the new charge in the cylinder.
Furthermore, the flame that starts combustion of the air-fuel mixture is quenched close to the relatively cold cylinder wall and as a result contains unburned fuel which is emitted with the exhaust gases during the exhaust stroke. In much the same way the fuel-air mixture is forced along the piston and in back of the top compression ring during the compression stroke. The fuel in this portion of the mixture does not burn during the main combustion process, and during the expansion stroke the fuel escapes and forms a layer of hydrocarbons on the relatively cold cylinder wall. During the exhaust stroke these hydrocarbons are scraped from the cylinder wall and join with the exhaust gases, enough of which survive post-quench oxidation to be a prime source of unburned hydrocarbon emissions.
When a single carburetor feeds a number of cylinders the fuel tends to go preferentially to certain cylinders at the expense of others. The resulting relative spread in the air-fuel ratio going to the various cylinders can be as much as 10-15%. In order for the lean cylinders to receive an ignitable mixture the rich cylinders must receive more fuel than can be efficiently burned. This problem is aggravated by the exhaust gas left in the combustion chamber from the previous cycle, which is substantially the same in volume for each cycle. Thus the ratio of exhaust gas to fuel mixture for the lean cylinders is greater than that for the rich cylinders.
The exhaust temperatures of present automobile engines are not high enough in the exhaust manifold during urban driving to oxidize the quenched hydrocarbons in the exhaust stream for adequate emission control.
In internal combustion engines powered by gasoline, exhaust CO and HC emissions could be reduced by increasing the ratio of air to fuel to the point where more air is present than is required for complete combustion so that the excess air could reduce the CO and HC to carbon dioxide and water. Maximum emissions of N.sub.x, however, would occur under such conditions. On the other hand, at any low air-fuel ratios the N.sub.x emissions could be reduced but high concentrations of CO and HC would be produced. Yet at extremely high air-fuel ratios where all three emissions could theoretically be low, the engine could be subjected to stalling and misfiring thereby causing poor performance.
The positive crankcase ventilation system (PCV) which works well on new engines has certain flaws. First, the PCV valve may become clogged with deposits and must be checked and perhaps replaced periodically. Second, when the engine parts are worn the amount of blow-by gases can overwhelm the PCV system, causing throwing of crankcase oil and other problems.